This invention relates to improvements in the front end elements, i.e., the shadow mask, mask frame, and inner shield, of a color cathode ray tube (CRT).
A color CRT, as shown in FIG. 1, generally comprises a glass envelope 1, in-line electron guns 3 emitting three electron beams 11, and a phosphor screen 5 containing red, green, and blue phosphors which emit visible light when excited by the electron beams 11. Electron guns 3 are located in the neck portion 2 of the envelope, while the phosphors, arranged in vertical stripes of cyclically repeating colors, are coated on the inner surface of the panel portion 4 of the envelope. Connecting neck 2 with panel 4 is the funnel portion 12 of the envelope. Electron beams 11 are deflected by magnetic fields produced by deflection yoke 10 surrounding a portion of the neck 2.
Near screen 5 is a shadow mask 6 having a plurality of vertically oriented rectangular apertures (not shown). Shadow mask 6 is attached to a mask frame 7 supported within the envelope by frame holders 8 which are releasably mounted on a plurality of panel pins 13 embedded in side walls of panel 4. An inner shield 9, also attached to mask frame 7, extends part of the way along funnel 12 toward electron guns 3, shielding the electron beams 11 from the effects of terrestrial magnetism. After emission from electron guns 3, electron beams 11 are accelerated, deflected by deflection yoke 10, and converged. They then pass through the apertures of shadow mask 6 to bombard phosphor screen 5, reproducing a color image.
The front end elements of the color CRT, i.e., shadow mask 6, mask frame 7 and inner shield 9, are conventionally made of aluminum-killed steel.sup.1 because it is easily etched (to make apertures) and easily formed into the necessary shapes for the front end elements. FNT 1 "Killed" steel, as is known in the art, is steel which has, while in the molten state, been caused to become quiet and free from bubbling by adding a strong de-oxidizing agent (such as aluminum) that combines with oxygen and minimizes reaction between oxygen and carbon during solidification. If the steel is incompletely de-oxidized, after solidification the outside portion is distinctly different in constitution from the interior of the ingot, and the material is known as "rimmed" steel.
Aluminum-killed steel is also easily coated with an oxide film, which helps to reduce reflection of the electron beams. Although conventional front end elements are coated with a black oxide film produced by a high temperature oxidation reaction, this oxide, which may be alpha Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4, adheres poorly to the base and occasionally spills, since it contains voids.
With the recent emphasis on personnal computer displays, teletext, and satellite transmission, front end elements made of aluminum-killed steel, especially shadow masks, have been unable to meet the high standards for resolution and "comfortable viewing." (Comfortable viewing is a term of art referring in part to the ability to discern fine characters and images on the screen, i.e., high resolution, and in part to a brighter picture produced by increasing beam current.) Increased beam current, of course, increases the amount of heat which must be dissipated by the front end elements.
When a color CRT is energized, electron beam current raises the temperature of the front end elements to anywhere from 303 K. to 373 K. At those temperatures, the shadow mask is deformed by thermal expansion, giving rise to what is called the "doming phenomenon." When this occurs, a misalignment comes about between the apertures of the shadow mask and the vertical stripes with which the apertures should be aligned. A color slippage phenomenon known as "purity drift" (PD) is the result. The smaller the apertures in the shadow mask, and the more closely spaced they are, and more serious is the color slippage problem. Since high resolution or "comfortable viewing" color CRTs use shadow masks with small, closely spaced apertures, the large thermal expansion coefficient of aluminum-killed steel makes it impractical for use with these color CRTs.
To overcome this problem, it has been suggested (in Japanese Publication No. 42-25446, Japanese Patent Disclosure No. 50-58977, and Japanese Patent Disclosure No. 50-68650) that shadow masks and other front end elements be made of an iron-nickel alloy which has a small coefficient of thermal expansion, such as Invar..sup.2 These alloys have the added advantage of being considerably harder than iron alone, so the closely-spaced apertures used in high definition television (HDTV) receiver shadow masks will not produce unaccetable weakening of the masks. FNT 2 Invar is a trademark with Registration Number 63,970.
Although alloys of iron and nickel are desirable because of their hardness and their small coefficient of thermal expansion, they have the disadvantage of low thermal conductivity, causing them to retain heat. Consequently, these alloys still exhibit an undesirable amount of color slippage when used as shadow masks or other front end elements in color CRTs.